Barbier, Edward B.; Koch, Evamaria W.; Silliman, Brian R.; Hacker, Sally; Wolanski, Eric; Primavera, Jurgenne H.; Granek, Elise F.; Polansky, Stephen; Aswani, Shankar; Cramer, Lori A.; Stoms, David M.; Kennedy, Christopher J.; Bael, David; Kappel, Carrie V.; Perillo, Gerardo; Reed, Denise J. 2008. Vegetation's role in coastal protection - Response. Science. Vol: 320. Pages 177. (Abstract) (Online version)
We are concerned about the assertion in the Report by E. B. Barbier et al. that vegetation reduces coastal damage during extreme events (â€œCoastal ecosystem-based management with nonlinear ecological functions and values,â€ 18 January, p. 321). Although the intended point was that ecosystem services do not linearly scale with habitat size, the conclusions drawn from the chosen example of wave attenuation are speculative. Figure S1, A to F, shows wave attenuation curves that are typical, with or without vegetation on the surface. Several other recent papers have also argued that vegetation reduces the impact of extreme events such as the Asian tsunami (1, 2) or Hurricane Katrina (3, 4). As in these studies, the evidence collected by Barbier et al. is based on correlation or visual description. Open in new tab CREDIT: JIM REED/CORBIS In the examples where vegetation appears to have protected the coast, there is no clarification as to the mechanism involved. It has been inductively proven that coastal plants can â€œengineerâ€ land elevation (topography and bathymetry) through succession, but this is a long-term process that occurs before the event (e.g., the sand dune example in Barbier et al.). It has not yet been determined whether vegetation can resist waves over a few meters in height during an extreme event (5). Thus, an important question is whether vegetation structure reduces coastal damage directly through wave attenuation or indirectly by altering the geography of the landscape. The difference has serious management and policy implications. If we emphasize direct structural value, we may end up with coastlines populated with invasive species or anthropogenic structures that kill more people than the waves themselves during extreme events (6, 7). And we still cannot guarantee that the surge (the real killer) or erosion [often enhanced by attenuation (8)] will be reduced.